Nervous system part 5

Cranial Nerves

Cranial Nerves• As many as 25• Named and numbered by Roman numerals from

anterior to posterior• Why abandon numerical nomenclature?– Development/ origin/ evolution– Variation in function

• Sensory• Motor• Mixed

– Organization– Clinical application• Superficial origin vs. deep origin

• Focus:– Origin/ location– Innervation– Type– Function– Notable location of cell bodies & nerve fibers– Clinical application

• Derived from:– Neurectoderm

• II, E– Neural crest

• Ganglia of V, VII, IX, and X– Ectodermal placode

• Ganglia of 0, I, VN, ALL and PLL• Taste bud components of VIII, VII, IX, and X

Sensory

• 0• I (Olfactory)• VN (Vomeronasal)• II (Optic)• E (Epiphyseal Complex)• P (Profundus)• ALL & PLL (Lateral Line)• VIII (Vestibulocochlear)

O, Terminal• 13th C.N. in humans• Last discovered C.N.• Gnathostome feature• Origin: ventral surface of forebrain• Innervation: nasal epithelium• Function: mediates responses to sex

pheromones (pheromone receptor)

I, Olfactory• No ganglia• Location: olfactory bulb• Innervation: olfactory epithelium (cell bodies)• Function: smell• Fibers poorly developed in birds & platypus• Vestigial in marine mammals• Clinical application: anosmia, loss of smell, due

to head injuries in which cribriform plate is injured

• Olfactory pathway:– Olfactory mucosa > Cribriform plate > Olfactory bulb >

Olfactory tract > Olfactory area of temporal lobe

VN, Vomeronasal• Innervation: vomeronasal organ

(chemoreceptor)

II, Optic• No ganglia• Origin: optic chiasma• Innervation: retina (cell bodies)• Function: vision• Decussation – crossing over to the opposite– Non-mammalian• All decussate and enter opposite side of brain

– Primates• Only axons from nasal side of retina decussate• Results in overlap of visual field (binocular vision)• Depth perception

• Visual pathway– Optic nerve > Optic chiasm > Optic tract >

Lateral geniculate nucleus of thalamus >Optic radiation > Primary visual area of cortex in occipital lobe

• Optic nerve a misnomer because retina arises from paired optic vesicles from telencephalon

• Lens derived from ectodermal placode• Clinical application: visual field defects, loss of

visual acuity, anopia

E, Epiphyseal complex• Innervation: pineal and parapineal

• Primitively fused with trigeminus as in sarcopterygians

• Discrete in gnathostomes (derived)• Innervation: skin of snout

P, Profundus

ALL and PLL, Lateral Line• Innervation: lateral line organs

(mechanoreceptors) of aquatic anamniotes• Up to 6 (3 ALL and 3 PLL)– Hagfish only 2 ALL, unicellular vs. multicellular

neuromast cells of lampreys and gnathostomes

VIII, Vestibulocochlear• Aka auditory nerve• Innervation: membranous labyrinth of inner ear• 2 branches:– Vestibular nerve

• Saccule, utricle, ampulla of semicircular canal• Vestibular ganglion

– Cochlear nerve• Cochlea, spiral organ (Organ of corti)• Cochlear or spiral ganglion

• Function: conveys impulses related to hearing and equilibrium

• Clinical application: – Injury to vestibular branch• Vertigo, feeling of rotation• Ataxia, muscular incoordination• Nystagmus, involuntary rapid eyeball movement

– Injury to cochlear branch• Tinnitus, ringing in the ears• Deafness

Motor

• III (Oculomotor)• IV (Trochlear)• VI (Abducens)• XI (Accessory)• XII (Hypoglossal)

III, Oculomotor• Origin: midbrain (mesencephalon)• Motor innervation– Levator palpebrae superioris– Superior, medial, inferior rectus– Inferior oblique

• Sensory innervation– Proprioceptors in eyeball muscles

• Autonomic innervation (parasympathetic)– Ciliary muscle for accomodation– Circular muscle of iris, pupil constriction

IV, Trochlear• Origin: midbrain• Motor innervation:– Superior oblique

• Sensory innervation:– Proprioceptors

• Few decussating fibers• Along with VI, smallest C.N.• No ganglion, cell bodies with nuclei in CNS

VI, Abducens• Origin: pons• Innervation– Lateral rectus– Proprioceptors

• Along with IV, smallest C.N.• No ganglion, cell bodies with nucleus in CNS

• Clinical application:– Strabismus, eyes do not fix on same object– Drooping upper eye lid– Movement of eyeball– Loss of accomodation– Diplopia, double vision– Fields of gaze

XI, Accessory• Origin: medulla• Innervation– Cranial root• Muscles of tongue, larynx, soft palate and pharynx

(few)• Mediates swallowing movements• Internal ramus – joins X (vagus) to be distributed to

autonomic ganglia in coelom (majority)

– Spinal root• External ramus – innervates trapezii and

sternocleidomastoid• Mediates movement of head and shoulders

• Lack dorsal roots• Derived from posteriormost branchiomeric

nerves of fishes and occipitospinal nerves• Clinical application: inability to raise

shoulders, difficulty to turn head

XII, Hypoglossal• Spinal nerve that became locked up in braincase• Derivative of the occipitospinal series of fishes• Develops embryonic dorsal root and ganglion

which regress• Origin: medulla• Innervation: – Motor

• tongue muscles (hyoglossus, styloglossus, lingualis)• Movement of tongue during swallowing and speech

– Sensory• Proprioceptors

• Join spinal nerves that innervate hypobranchial muscles (geniohyoid, sternothyroid, sternohyoid, throhyoid)

Mixed

• Carry both sensory and motor fibers• All carry somatic motor, sensory, and

autonomic motor components except V (somatic and sensory)

• Innervate pharyngeal arches (branchiomeric nerves)

• V (Trigeminal)• VII (Facial)• IX (Glossopharyngeal)• X (Vagus)

V, Trigeminal• Lacks fibers for taste• Lacks autonomic (visceral) component• 3 branches all with sensory, only mandibular

with motor– Opthalmic• Discrete profundus nerve• Seen in agnaths, sarcopterygians, actinopterygians,

tetrapods• Innervates skin, upper eyelid, eyeball, lacrimal glands,

nasal cavity, nose, forehead, anterior scalp

– Maxillary• Infraorbital nerve of fishes• Innervates nasal mucosa, palate, pharynx, upper teeth,

upper lip, lower eyelid

– Mandibular• Sensory innervation

– Anterior tongue (lateral lingual swellings), lower teeth, skin over mandible, cheek, side of head

– Conveys impulses for touch, pain, temperature sensation

• Motor innervation– Muscles of mastication (mandibular arch)– Functions in chewing

• Clinical application:– Paralysis of chewing muscles– Loss of touch, pain, and temperature sensation in

lower part of face– Dental anesthetic drugs • Maxillary nerve for upper teeth• Mandibular nerve for lower teeth

VII, Facial• Innervation– Sensory

• Taste buds– Extend all the way to tail in fishes– Phrayngeal branch in fishes supply taste buds of pharygeal

mucosa– Anterior tongue of tetrapods (glandular field)

• Proprioceptors • Functions in taste and proprioception• Cell bodies located in facial or geniculate ganglion

– Autonomic (Parasympathetic)• Innervate lacrimal, sublingual, submandibular, nasal, and

palatine glands• Secretion of saliva and tears

– Motor• Innervates facial, scalp, and neck muscles• Function in facial expression

• Clinical application– Loss of taste– Decreased salivation– Loss of ability to close eyes– Bell’s palsy, paralysis of facial muscles

IX, Glossopharyngeal• Innervates two successive arches (shark)• Branches– Pretrematic • Monitors condition of gill filaments in anterior wall og a

gill chamber

– Posttrematic• Operate muscles of arch• Monitors condition of gill filaments in posterior wall

– Pharyngeal• Extend as branch of pretrematic to palate• Monitors pharyngeal mucosa

• Innervation– Sensory• Posterior taste buds (3rd arch mesenchyme)• Proprioceptors• Functions in taste, touch, pain, and temperature

sensation from posterior tongue• Functions in proprioception in swallowing muscles

– Motor• Stylopharyngeus • Elevates larynx during swallowing

– Autonomic (parasympathetic)• Parotid gland• Secretion of saliva

X, Vagus• Branchial trunks innervate 2nd-5th gill chamber

as in IX– Pretrematic sensory to pretrematic demibranch– Posttrematic sensory and motor to posttrematic

demibranch– Both sensory for general sensation and taste

• Ramus visceralis (autonomic) to viscera– Major branch of vagus in tetrapods– Accompanied by visceral sensory fibers

• Ramus lateralis (autonomic) to lateral-line canal

• Innervation– Sensory

• Taste buds in epiglottis and pharynx• Proprioceptors in neck and throat muscles• Visceral sensory receptors• Taste, touch, pain, and temperature sensation, proprioception• Sensation from viscera

– Motor• Throat and neck muscles• Swallowing, coughing, voice production

– Autonomic (parasympathetic)• Smooth muscle of GIT from esophagus to large intestine, gall

bladder• Cardiac muscle• GIT glands• Contraction and relaxation of GIT muscles, slowing of heart rate,

digestive fluid secretion

Tongue

• Oral mucosa: V, VII, IX, and X from fish to humans

• Tongue innervation of V, VII, IX because derived from I, II, III, pharyngeal arches

• Tongue muscles (hypobranchial) myotomal receive XII

• 4 C.N. innervate tongue but only 3 can be traced because VII unites with V